1. Technical Field of the Invention
The present invention relates to a method and apparatus for decoding CDMA (code division multiple access) signal, and particularly to a method and apparatus for decoding a direct sequence spread spectrum signal which is modulated by pseudo noise codes of which rate are higher than the information signal rate.
2. Description of the Prior Art
Amplitude and phase fluctuation occur in a mobile communication system, due to Rayleigh fading caused by a relative movement between a mobile station and a base station. One of the methods for estimating and compensating the fading distortion is to use pilot symbols which are inserted between information symbols with a constant period. In this case, the phases of the pilot symbols are known.
Concretely, a pilot symbol of which transmission phase is known is inserted per a plurality of information symbols on a communication channel, thereby estimating a transmission channel on the basis of the phase of the received pilot symbol. The fluctuation of the transmission channel between the pilot symbols is estimated and compensated by measuring the amplitude and phase in each path in a prescribed period of the information symbol between the front pilot symbol and the rear pilot symbol.
Further, the transmission channel is estimated more accurately by using a lot of pilot symbols for a lot of time slots, as disclosed in JP 10-51424 A (1998) and JP 11-186990 A (1999).
FIG. 4 is a block diagram of the receiver as disclosed in JP 11-186990 A (1999). FIG. 5 is a block diagram of multi-pilot block channel estimation unit 8 as shown in FIG. 4. As shown in FIG. 4, matched filter 2 despreads the data inputted from reception data input terminal 1, on the basis of receiving timing of each multi-path. “2k” slots from the (n−(k−1))−th slot to the (n+k)−th slot in the despread data are stored in memory 3. Here, “n” is a non-negative integer and “k” is a positive integer.
The slot comprises a pilot block, TPC (transmission power control) information and transmission data. Further, the pilot block includes a plurality of pilot symbols, for example, three symbols as shown in FIG. 5.
Memory 3 may stores only the pilots blocks of the received slots. The positions of the pilot symbols are detected by slot position detection unit 4 which picks up the pilot blocks from memory 3. Then, pilot symbol averaging unit 5 averages a plurality of received channels at a plurality of the pilot symbols in a slot. The amplitude of the received signal varies slot by slot, because of the TPC, even when there is not any fluctuation on the transmission channel. Therefore, the amplitude variation due to the TPC may be compensated. The transmission timings by the pilot blocks are arranged by delay unit 6.
Multiple-pilot block channel estimation unit 8 multiplies the transmission channel estimation value of each pilot block by weight coefficient and adds them. The result of the addition is a complex channel estimation value. Then, multiplier 10 multiples the data from memory 3 by the complex conjugate of the complex channel estimation value in order to compensate the fading phase fluctuation of each information symbol. The signals outputted from multiplier 10 is combined with other RAKE fingers by RAKE combining unit 11. The result of the RAKE combining is outputted from decoded data output terminal 12 of RAKE combining unit 11.
On the other hand, the output from delay unit 6 is inputted as a transmission channel estimation value of the TPC symbol into TPC command determination unit 9 in order to compensate the fluctuations of fading phase due to TPC.
The operation of the above-mentioned conventional CDMA signal decoding is explained. Output 301 from pilot symbol averaging unit 5 is an average of the received channels at a plurality of pilot symbols. The average means a transmission channel estimation value for each pilot block. The average for each pilot block, in other words, a complex fading trajectory estimation value is expressed byξ1(n+k), . . . , ξ1(n+1) ξ1(n), . . . , ξ1(n−(k−1)).
The pilot blocks are inputted into multipliers 385, 390,395, 396 and 397 through pilot symbol averaging unit 5 and delay units 320, 325, 330, 335 and 340, respectively. Further, output 301 from pilot symbol averaging unit 5 is inputted into multiplier 380. Multipliers 380385390, 395, 396 and 397 multiply output 301 by weights αk, α2, α1, α0, α1, α(−k+1). The outputs from the multipliers are added by adder 398 in order to obtainξ1(m,n) which is a transmission channel estimation value of the m-th information symbol in the n-th slot.
The conventional method for estimating the transmission channel is shown in FIG. 6. As shown in FIG. 6, the transmission channel estimation values obtained by using the pilot symbols are plotted on the I-Q coordinates, where I is an in-phase component and Q is a quadrature component. Each point is plotted by a single value estimated in a slot. The trajectory varies greatly at a fast fading, while the trajectory varies a little at a slow fading, as shown in FIG. 6.
Weights αk, α2, α1, α0, α−1, α(−k+1) are constant in FIG. 6 in order to improve the accuracy of estimation against thermal noise and interference signal.
Although the slow fading when the transmission channel is stable is followed well, it is not easy to follow the fast fading, when the number of slots “2k” increases and the weights are of the same order.
On the contrary, when the number of slots “2k” decreases and the weights are set up to be approximately zero except specific slots, it is not easy to follow the slow fading, because the thermal noise and interference do not become negligible, although the fast fading can be followed well.